Electrodeposition of ruthenium



United States Patent 3,123,544 ELECTRODEPOSITION 0F RUTHENIUM JoyceCresswell Blake, London, England, asslgnor to The International NickelCompany, Inc., New York, N.Y., a corporation of Delaware No Drawing.Filed Apr. 10, 1962, Ser. No. 186,318 Claims priority, application GreatBritain Apr. 18, 1961 7 Claims. (Cl. 204-47) The present inventionrelates to electrodeposition and, more particularly, to theelectrodeposition of ruthenium.

Hitherto, ruthenium has usually been electrodeposited cathodically froma fused electrolyte. The electrolyte has consisted of a fused mixture ofsodium and potassium cyanides that has been charged with ruthenium byalter nating current electrolysis between ruthenium electrodes. Thisgives satisfactory, coherent coatings up to 0.0015 inch thick, but hasthe disadvantage that a temperature of about 500 C. must be used tomaintain the bath molten.

This disadvantage can be overcome by the use of an aqueous electrolyteand it has been proposed to use an acid aqueous solution of a rutheniumsalt containing a nitroso group. It is, however, difiicult to make suchsolutions with reproducible properties. Although attempts were made toovercome the foregoing disadvantage, none, as far as I am aware, wasentirely successful when carried into practice commercially on anindustrial scale.

It has now been discovered that by the use of a special plating methodincluding a novel electroplating bath, advantageous rutheniumelectrodeposits can be produced from an aqueous medium.

It is an object of the present invention to provide a novel process forthe electrodeposition of ruthenium.

Another object of the invention is to provide a novel rutheniumelectroplating bath.

Other objects and advantages will become apparent from the followingdescription.

According to the invention, ruthenium is electrodeposited from anaqueous acidic solution of a soluble tetrasulphamato nitrosyl ruthenate,preferably the sodium salt. Such solutions can be prepared withsatisfactory reproducibility from sodium tetranitro nitrosyl ruthenate,which is precipitated as fine yellow needles of definite composition bytreating a boiling solution of ruthenium chloride in normal hydrochloricacid with twice the theoretical amount of sodium nitrite and allowingthe reaction mixture to cool. On warming an aqueous solution of thecrystals to 100 C. with solid sulphamic acid, nitrogen is evolved in anamount that corresponds, when reaction is complete, to the replacementof all four nitro groups by sulphamato groups. The product of thereaction is a clear deep-red solution which gives very reproducibleresults when used as an electrolyte for the deposition of ruthenium.

The ruthenium content of the solution used for the electrolysis isadvantageously about 2.5 to about 20 grams per liter (g.l.). At lowerconcentrations than 2.5 g./l., ruthenium tetroxide is evolved at theanode and we prefer to use a solution containing about g./l. ofruthenium. Increasing the ruthenium concentration above this, decreasesthe cathode efliciency and increases both the cost of the bath and thelosses of ruthenium by drag-out.

The acidity of the solution is not critical, but advantageously from 10to 50 g./l. of free sulphamic acid is present. This may be partly orwholly replaced by sul-. phuric acid. The remainder of the bath isessentially water.

The bath can be used from room temperature up to boiling point. Thecathode efficiency of the bath increases with temperature and a suitableworking temperature range is from 50 C. to 90 C., advantageously about3,123,544 Patented Mar. 3, 1964 "ice 70 C. Increasing the currentdensity decreases the cathode efiiciency and advantageously the cathodecurrent density is about 2 to about 8 amperes per square decimeter(amps./dm.

By way of example, a bath containing 5 g./l. of ruthenium as sodiumtetrasulphamato nitrosyl ruthenate and 30 g./l. of free sulphamic acidwas used to plate copper discs 2 square inches in total area which hadpreviously been given a thin electrodeposit of copper from an acidcopper sulphate bat'h followed by a flash coating of gold or palladium.The ruthenium plating bath was maintained at a temperature of 70 C. andruthenium was electrodeposited on the discs at a current density of 8amps./dm. using insoluble anodes of platinum foil. The deposition ratewas about 0.00015 inch/hour and the cathode efficiency was 5%. Thedeposits were bright and substantially crack-free up to a thickness of0.0002 inch. Their surfaces showed a very faint crack pattern but thisdid not show up when the deposits were sectioned. At a thickness of0.00045 inch the deposits were bright and frosted.

At a thickness of 0.0001 inch, one or" the deposits had an internaltensile stress of 25.7 tons/sq. inch and at a thickness of 0.0005 inchit had a hardness of about 900 DPN.

A similar bath operated under similar conditions but using a currentdensity of 4 amps./dm. electrodeposited ruthenium on a cathode at adeposition rate of 0.00009 inch/ hour with a cathode efliciency of 5.5%.

Although the present invention has been described in conjunction withpreferred embodiments, it is to be understood that modifications andvariations may be resorted to without departing from the spirit andscope of the invention, as those skilled in the art will readilyunderstand. Such modifications and variations are considered to bewithin the purview and scope of the invention and appended claims.

I claim:

1. A process for the electrodeposition of ruthenium which comprisesoathodically depositing ruthenium from an aqueous acidic bath containingat least about 2.5 grams per liter of ruthenium dissolved therein as atetrasulphamato nitrosyl ruthenate at a cathode current density of about2 to about 8 amperes per square decimeter.

2. A process as defined in claim 1 wherein the bath contains up to about50 grams per liter of an acid selected from the group consisting ofsulphuric acid, sulphamic acid and mixtures thereof.

3. A process as defined in claim 2 wherein the bath contains about 2.5to about 20 gna-ms per liter of ruthenium and is maintained at atemperature of about 50 C. to about C.

4. A process as defined in claim 2 wherein the bath contains at leastabout 10 grams per liter of acid.

5. A ruthenium electroplating bath comprising an aqueous acidic solutionof at least about 2.5 grams per liter of ruthenium as a tetrasulphamatonitrosyl ruthenate.

6. A ruthenium electroplating bath comprising an aqueous acidic solutionof about 2.5 to about 20 grams per liter of ruthenium as atetrasulphamato nitrosyl ruthenate.

7. A ruthenium electroplating bath as defined in claim 6 which containsabout 10 to about 50 grams per liter of an acid selected from the groupconsisting of sulphuric acid, sulphamic acid and mixtures thereof.

References Cited in the file of this patent UNITED STATES PATENTS2,600,175 Volterra June 10, 1952 FOREIGN PATENTS 466,126 Great BritainMay 18, 1937

1. A PROCESS FOR THE ELECTRODEPOSITION OF RUTHENIUM WHICH COMPRISESCATHODICALLY DEPOSITING RUTHENIUM FROM AN AQUEOUS ACIDIC BATH CONTAININGAT LEAST ABOUT 2.5 GRAMS PER LITER OF RUTHENIUM DISSOLVED THEREIN AS ATETRASULPHAMATO NITROSYL RUTHENATE AT A CATHODE CURRENT DENSITY OF ABOUT2 TO ABOUT 8 AMPERES PER SQUARE DECIMETER.